The problem of the detonation of a charge at a predetermine distance from a target has in the past involved radio frequency signal generation and detection, reticle type position indicating systems and various chopping techniques. Active systems, those which transmit energy and receive return energy, are not desireable because the energy radiated may be detected and a jamming technique may be utilized once the existence of an incoming ordinance device is recognized.
By way of background, systems which sense an in-focus condition have been disregarded as not being sensitive enough to provide accurate distance measurement without complicated parallax processing. This lack of satisfaction with the utilization of focus alone as the distance determining element is described in U.S. Pat. No. 3,348,050 issued to R. V. Bez on Oct. 17, 1967. In this patent it is said that "the major difficulty in determining range by this method (an in-focus indicating method utilizing a moveable chopper grid) arises because the degree of modulation changes very slowly as the chopper grid is moved in either direction from the focal plane, and therefore it is difficult to servo on the maximum point". In Bez's patent a chopper grid is rotated in front of a detector and the percentage of modulation is sensed, with an in-focus condition indicated by a maximum in the amplitude of the signal indicating modulation percent.
It is the recognition that this statement refers to relatively coarse grids where the grid lines occupy no more than 10% of the number permitted by the resolution of the lens which leads to the subject invention. In the prior art systems the patterns on the choppers do not represent a single spatial frequency across the entire extent of the chopper. Moreover, since the outer patterns on the chopper described in the Bez patent revolve faster than inner patterns there is no one temporal modulation frequency to which the systems described in the Bez patent are set. This results in a lack of resolution or sensitivity. On the other hand, the subject system to be described uses a stationary grid set to a single spatial frequency to achieve exceptional sensitivity. As will be described, the optimum frequency for this grid is related to the resolution of the optical system and is the frequency at which there is a maximum difference between the in-focus modulation transfer function (MTF) curve for the optical system utilized and the MTF curve associated with the maximum focus offset for a given system accuracy.
The modulation transfer function is perhaps the most accurate way of defining the resolution of an imaging system as it takes into account not only the ability of an imaging system to resolve adjacent lines at an object plane but also the intensity of the resolved lines. By setting the grating to a single frequency as described above, one maximizes the response of the system based on the difference in resolution at the in-focus condition and resolution at a predetermined out-of-focus condition related to range accuracy.
Once the frequency of the grating is set by appropriate patterning of the cross-correlation grating, very small defocusing errors result in very large signal differences and the difference between the in-focus, and out of focus condition is magnified. The subject system also uses a nutating scan to achieve a constant temporal modulation frequency everywhere within the field of view. This minimizes the required video bandwidth, allowing reduction of the electrical noise in the system.
For example, utilizing a diffraction limited optical system, one whose resolution is impaired only by the finite wavelength of light, and assuming that it is desirable to detonate a charge at, for instance, 10 meters .+-.0.01 meter from a target, in a given system this tolerance may correspond to a shift of the axial position of the image plane from the in-focus image plane (the image plane established by images at exactly 10 meters in this case) a distance of only .+-.3 wavelengths of the light detected by the system. Having determined that an accuracy of .+-. 0.01 meters (corresponding to a 3 wavelength offset) is acceptable, the spatial frequency of the cross-correlation grating is set to correspond to that frequency at which the greatest difference exists between the 3 .lambda. modulation transfer function (MTF) and the in-focus MTF. This can be accomplished by inspection of the particular MTF curves for a given optical system.
It will be appreciated that for diffraction limited optical systems the rate change of the amplitude of the output signal vs. spatial frequency is greatest at the higher spatial frequencies. By setting the spatial frequency of the cross-correlation grating at the point of maximum signal variation, the grating in effect chooses the high spatial frequency portion of the spectrum. As a result, objects having well defined edges such as aircraft, naval vessels, etc., are distinguished from their surroundings which may include clouds having diffuse edges or ocean waves having no easily discernable edges whatsoever.
It will be appreciated that systems in the prior art have spatial frequencies which are exceptionally broadband as they utilize rotating choppers or chopping discs having patterns which do not define a single spatial frequency. This was thought to be desirable in order to preclude jamming. However, it has been found that by the utilization of a single spatial frequency grating and that by appropriate setting of the spatial frequency, a sensitive narrow band system is achieved which minimizes the effectiveness of countermeasures. In the subject system the spatial frequency can be easily changed by changing the grating and different spatial frequency gratings can be easily inserted in different ordinances for countermeasure resistance. The subject system therefore not only discriminates against low spatial frequency objects but also acts as a very sharp filter for countermeasure rejection.
In one embodiment the subject invention includes an optical imaging system with an optical wedge rotated about the optical axis of the system and with a cross-correlation grating fixedly mounted at the image plane of the optical system for an object at a desired distance. The grating in one embodiment includes a number of apertures arranged at the apices of a hexagonal pattern. This type grating provides a nearly constant temporal modulation (chopping) rate for the nutating image over the entire field of view, permitting use of a narrow band video amplifier for optimum noise and countermeasure rejection. A field lens is utilized in back of the cross-correlation grating to concentrate the light from the entire grating onto a single detector. The output of the detector is amplified and scan or temporal modulation frequency components are eliminated at a bandpass filter which is set at a frequency higher than the temporal modulation frequency. In this regard, in one embodiment, the passband filter is set to the spatial frequency of the grating times the image velocity (units of distance/sec.) where the units of distance are the same units of distance as those used in defining the spatial frequency. The output of the filter is thus a signal representing the degree of modulation of the light from the grating. The output from the bandpass filter is rectified to provide a DC signal whose level is proportional to the amount of modulation of the light through the grating. An adaptive threshold is set in one embodiment such that when the level of the DC signal exceeds the threshold indicative of maximum modulation, an in-focus condition is sensed and a charge is detonated.
What has therefore been provided is an exceptionally simple and extremely accurate range determining system which may be utilized on a large variety of projectiles or missiles or other optical systems, such as cameras. In the case of spinning projectiles or missiles it is preferrable to despin the fuze to keep the image from rotating on the grating in order to minimize video bandwidth. On missiles and projectiles which are not spin stabilized, rotation of the wedge is easily accomplished.
By utilizing diffraction limited optics and by careful choice of the spatial frequency of the cross-correlation grating, practical accuracies of .+-.1 meter for ranges of 100 meters can be achieved. In the theoretical limit, with perfect defraction limited optics, this error can be cut to .+-.1 centimeter in the best case. Thus while rotating chopper and modulation detection systems have been proposed in the past, by use of a fixed grating, a nutating image and proper selection of the single spatial frequency for the grating, at least an order of magnitude improvement in resolution is now achieveable.
It is therefore an object of this invention to provide an improved range determining method and apparatus.
It is another object of this invention to provide an improved passive optical fusing mechanism utilizing a nutating image movement or circular scan and a cross-correlation grating which not only provides for selection of a single spatial frequency but also selects the spatial frequency on the basis of the maximum MTF curve variation at the desired range accuracy.
It is a further object of this invention to provide for the nutation of an image at the image plane of an optical system at which a cross-correlation grating is located.
It is a further object of this invention to provide an improved cross-correlation grating for passive optical range finding equipment or cameras.